PROJECT SUMMARY: Our lab recently co-discovered the COPA syndrome, an autoimmune disease caused by dominant mutations in the Coatomer subunit alpha (COPA) gene that manifests as inflammatory arthritis and interstitial lung disease (ILD). COPA encodes the COPA subunit of coat protein complex I (COPI). COPI is engaged in the retrograde movement of proteins from the Golgi to the ER and is a vital component of a cell's trafficking machinery. Patients with the COPA syndrome develop high-titer antinuclear or anti-neutrophil cytoplasmic autoantibodies, immune-mediated kidney disease and an increase in CD4+ T helper 17 (Th17) cells, an immune cell subset implicated in autoimmunity. Some patients have findings consistent with rheumatoid arthritis RA including rheumatoid factors or autoantibodies to cyclic citrullinated peptides. All COPA syndrome patients eventually develop interstitial lung disease (ILD) in combination with their arthritis. Generalized immunosuppressive drugs such as mycophenolate have been used to treat patients with limited success, although the ILD typically progresses on treatment and leads to end stage lung fibrosis. We generated a germline point mutant knock-in mouse bearing the exact same E241K mutation as COPA syndrome patients. Preliminary data demonstrates that CopaE241K/+ mice spontaneously develop mononuclear lung infiltrates and an examination of peripheral lymphoid tissues reveals a significant increase in effector memory T cells. Detailed study of developing thymocytes shows a significant increase in mature CD8+ and CD4+ single positive (SP) cell populations, findings that suggest alterations in thymocyte development or selection. CopaE241K/+ mice bred to a T cell receptor transgenic mouse system revealed a defect in the negative selection of CD4+ T cells. Interestingly, reciprocal bone marrow chimera experiments map the selection defect to E241K COPA expression in the thymic stroma. Thus, we hypothesize that a critical precursor to autoimmune disease in the COPA syndrome is aberrant thymocyte selection caused by mutant COPA expression in the thymic epithelium. We propose to pursue this hypothesis by 1) Determining the role of mutant COPA in positive and negative selection of CD4+ and CD8+ T cells 2) Defining the role of the thymic stroma on thymocyte development in CopaE241K/+ mice and 3) Defining the autoimmune features of CopaE241K/+ mice and COPA syndrome subjects. Through our study we seek to determine the role of thymic tolerance in the pathophysiology of the COPA syndrome and establish CopaE241K/+ mice as a preclinical model for the disease. Because the COPA syndrome shares features with other inflammatory joint disorders, our study may provide novel insight into how impaired vesicular trafficking contributes to the pathogenesis of arthritis and ILD by altering the negative selection of autoreactive T cells and causing a defect in central immune tolerance.